Silicon wafer etching is an important act in the manufacturing of semiconductor devices. As the size of semiconductor device structures decreases, it is desirable to form narrow trenches in silicon, in which the silicon etch rate and the resulting shape of the trench are controlled. Various wet etch chemistries have conventionally been used to etch silicon. For example, when an isotropic etch is desired, silicon is etched with a mixture of nitric acid (HNO3) and hydrofluoric acid (HF). In some cases, water is used to dilute the etchant solution. Typically, acetic acid (CH3COOH) is used as a buffering agent for such an etchant solution.
The drawbacks of using solutions of HNO3 and HF to etch silicon include the difficulty of controlling the consumption of reactants and the evolution of nitrous oxides as they dissolve into the etchant solution. The nitrous oxides dissolved in the etchant solution have a tendency to “poison” the etchant solution by saturation, affecting subsequent etches and etch rates. The process is difficult to control when forming uniform shapes in the silicon in which the size of the etched shape must be precisely controlled. When isotropically etching with HNO3 and HF, if the solution is not distributed uniformly across the silicon, the resulting etch may be non-uniform. Because the oxidizer (HNO3) and the etchant (HF) are in the same solution, it is difficult to finely control how much oxidation of the silicon and subsequent etching takes place at any given time. Once the silicon is oxidized, the silicon oxide may immediately be etched by the same solution and the process of oxidation and etching can repeat itself very rapidly. Thus, these solutions may result in non-uniform etches, undercutting, and pitting of the silicon. Furthermore, because HNO3 and HF etchant solutions require long exposure times to etch the silicon, the process has a tendency to damage other exposed materials, such as liners, on the semiconductor device structures. The use of conventional HNO3 and HF etchant solutions result in undercutting of the silicon and the formation of non-uniformly sized trenches, as shown in FIG. 1. In addition, the conventional HNO3 and HF etchant solutions may remove other exposed materials, such as silicon oxide or silicon nitride, in addition to the silicon. Etchant solutions other than HNO3 and HF, such as alkaline chemistries including KOH, NH4OH, TMAH, or NaOH, result in the formation of sharp corners in the silicon, as shown in FIG. 2, due to a preferential etch in the direction of the crystalline orientation of the substrate. Furthermore, the HNO3 and HF may remove too much silicon, such that adjacent trenches may touch, resulting in poor isolation of features formed in the trenches. The sharp corners increase the likelihood of electrical implications in the semiconductor device and can lead to component failure.
It would be desirable to be able to create substantially uniformly shaped trenches in a semiconductor device structure while controlling the selectivity of the etchant solution relative to other exposed materials. Further, it would be desirable to use an etchant solution by which the size and shape of the trenches may be accurately controlled.